Diagnosis
This is almost always a foreign object caught in the wash arm assembly or the wash pump impeller. A utensil or piece of glass is the usual suspect.
How to Fix It
Remove the lower rack. Manually rotate the lower wash arm. Check for obstruction. Remove the filter assembly (turn the blue knob counter-clockwise and lift out). Shine a flashlight into the sump area. Use needle-nose pliers to remove any debris from the impeller at the bottom.
If That Didn't Work
The wash motor itself could be failing. Listen closely. A consistent electrical hum points to the motor. A mechanical grinding points to debris or a damaged wash arm hub.
Heads Up
Always check for debris first. Running it with something jammed in the pump can burn out the motor. Remove large items from the bottom of the tub before every load.